1. Field of the Invention
The present invention relates to a reflow-plated member and a manufacturing method therefor, and more specifically, to a method for manufacturing a reflow-plated member which is satisfactory or excellent in wettability by solder, bendability, and heat resistance.
2. Prior Art
Plated members may be obtained by coating the surface of a base material made of Cu or Cu alloy with Sn or Sn alloy. They are high-performance conductors which efficiently combine the good electrical conductivity and mechanical strength of Cu or Cu alloy and the corrosion resistance and solderability of a coating layer of Sn or Sn alloy, and are used in a wide variety of applications, including various electrical and electronic appliance parts, such as terminals, connectors, lead wires, etc., and wire cables.
Conventionally, the connectors are fabricated by plating the surface of a strip of Cu alloy with Sn or Sn alloy and punching pieces in a predetermined shape from the plated strip. In this case, however, scraps are inevitably produced in the punching process. Recently, therefore, scrap-free pin connectors (pin-grid arrays) have been developed and started to use, as their pin material, a square wire (base material) of Cu alloy plated with solder (Sn--Pb alloy).
Melt plating and bright electroplating are prevailing methods of preparing the material of this kind.
The melt plating is a method in which a base material of Cu or Cu alloy is continuously run in molten Sn or Sn alloy so that its surface is coated with a layer of Sn or Sn alloy.
Although this method ensures relatively low manufacturing cost, it involves a problem that the resulting layer is highly irregular in thickness, entailing a large variation in thickness. Moreover, a thick layer of a Cu--Sn intermetallic compound is liable to be formed on the interface between the base material (Cu or Cu alloy) and the layer, resulting in the following awkward problems.
If the Cu--Sn intermetallic compound layer, a rigid layer itself, is thick, the resulting plated member sometimes may be fractured when it is bent in machining, for example. The Cu--Sn intermetallic compound, moreover, is a chemically stable substance itself. If this layer is too thick, therefore, the compound cannot easily react to solder when the plated member is soldered, so that the solderability of the layer is lowered.
On the other hand, the bright electroplating has frequently been used as a method which enables formation a thin layer of uniform thickness. According to this method, the surface of the layer is smoothed and brightened by loading a plating bath of Sn or Sn alloy with additives, such as benzylideneacetone, cinnamaldehyde, or other brightener and glue, gelatin, .beta.-naphthol, or other smoothing agent.
In the case of the bright electroplating, however, the additives are occluded at the boundaries between deposited crystal grains which constitute the formed plated layer, so that the binding power between the crystal grains is lessened. As a result, a Cu component of the base material and the like freely diffuse at the grain boundaries, and the aforesaid Cu--Sn intermetallic compound layer is formed thick in the plated layer, so that the bendability and solderability of the plated layer are liable to be lowered. The additives make the crystal grains more minute, thereby enlarging distortion at the grain boundaries. This accelerates diffusion of the Cu component and the like, which entails the aforesaid problems.
If the deposited crystal grains become more minute, discoloration advances starting from the grain boundaries, and is accelerated by change in properties of the additives which are occluded at the grain boundaries or adsorbed by the plated layer surface.
If the binding power between the crystal grains is lessened by the occlusion of the additives, moreover, the wear resistance of the plated layer is lowered. If the plated member is touched by a working tool or the like to be subjected to external force when it is worked, for example, the plated layer is liable to pulverize and separate from the base material. In some cases, furthermore, the additives may grow whiskers, as well as the crystal grains, so that the resulting plated member cannot be a reliable electrical or electronic appliance part.
In order to solve the problems of the melt plating and bright electroplating described above, a reflowing method has been developed and is widely-used now.
In this reflowing method, a plated layer is first formed on the surface of a base material by electroplating, using a plating bath of Sn or Sn alloy which is loaded with only a smoothing agent without containing any of the aforesaid brighteners. Then, the resulting plated member is continuously run in an furnace which is adjusted to a predetermined temperature, whereby the plated layer is melted and brightened.
According to the reflowing method described above, the additive (smoothing agent) occluded at the grain boundaries during the plating process is thermally decomposed and removed in a reflowing process in the next stage, so that the binding power between the crystal grains is enhanced. In the reflowing process, moreover, stress strain at the grain boundaries is eased. Thus, plated members (reflow-plated members) manufactured in the reflowing process surpass ones which are manufactured by the melt plating or bright electroplating in bendability, solderability, wear resistance, etc.
Even according to this reflowing method, however, a relatively thick Cu--Sn intermetallic compound layer may be formed on the interface between the base material and the plated layer, depending on the reflowing conditions, so that the bendability or solderability may possibly be lowered. After the reflowing process, moreover, the crystal grains in the reflowed-plated layer may become so coarse that the wettability of the layer surface by solder is worsened. After the plated layer is reflowed, furthermore, it cannot enjoy a satisfactory wear resistance, and may pulverize, though only slightly, when it is rubbed.
With the recent progress of miniaturization of electrical and electronic appliance parts, the reflow-plated members have come to require further improvement in various properties, such as formability, springiness, and electrical conductivity, and also a satisfactory heat resistance such that stable functions can be fulfilled even under severe temperature conditions.
However, the molten state of the plated layer (Sn or Sn alloy) formed on the surface of the base material (Cu or Cu alloy) varies depending on the reflowing conditions.
If the running speed of the plated member in the reflowing process is too low, for example, the plated layer melts so that its fluidity increases. If the running speed is too high, on the other hand, melting of the plated layer cannot advance, so that the reflowed layer cannot be brightened.
The thickness of the reflowed-plated layer becomes irregular, that is, one portion of the reflowed-plated layer becomes thicker than another, if the running speed of the base material during the reflowing process is so low that the fluidity of the molten plated layer is increased, if the molten state of the plated layer lasts for a long period of time, or if the base material oscillates violently during the reflowing process. After prolonged actual use of the reflow-plated members at high temperature, the Cu component of the base material or Cu--Sn intermetallic compound layer may diffuse to the surface of the thinner portion of the reflowed-plated layer, thereby changing its color. These reflow-plated members are poor in heat resistance.
In the case where the target reflow-plated member is a solder-plated square wire whose base material is the aforementioned square wire, in particular, the molten layer is caused by surface tension to flow from the corner portions of the square wire to the flat portions thereof during the reflowing process. As a result, the reflowed-plated layer at the corner portions becomes so thin that the discoloration is liable to occur. Inevitably, therefore, the heat resistance of the solder-plated square wire would be worsened.